So if you woke up in the Paleozoic, you probably wouldn't recognize Earth. How about if you woke up in the Mesozoic? In some ways, the planet would look a lot more like it does today. Animals would fill the niches you're used to seeing animals fill. But if you looked closely, you'd see that the animals are mostly all reptiles. And some of them may be interested in having you for dinner!

Mesozoic Life

With most niches available after the mass extinction, a great diversity of organisms evolved. Mostly these niches were filled with reptiles.

Climate alternated between cool, warm, and tropical, but overall the planet was much warmer than today. These conditions were good for reptiles. Surprisingly, there was more oxygen in the Mesozoic atmosphere than there is today.

Marine Life

Tiny phytoplankton arose to become the base of the marine food web. At the beginning of the Mesozoic, Pangaea began to break apart, so more beaches and continental shelf areas were available for colonization by new species of marine organisms. Marine reptiles colonized the seas and diversified. Some became huge, filling the niches that are filled by large marine mammals today.

Terrestrial Life

On land, seed plants and trees diversified and spread widely. Ferns were common at the time of the dinosaurs (
Figure
below
). The earliest known fossil of a flowering plant is from the Cretaceous, 125 million years old.

The earliest known fossil of a flowering plant is this 125 million year old Cretaceous fossil.

Dinosaurs

Of course the most famous Mesozoic reptiles were the dinosaurs (
Figure
below
). Dinosaurs reigned for 160 million years and had tremendous numbers and diversity. Species of dinosaurs filled all the niches that are currently filled by mammals. Dinosaurs were plant eaters, meat eaters, bipedal, quadrupedal, endothermic (warm-blooded), exothermic (cold-blooded), enormous, small, and some could swim or fly.

Some examples of Mesozoic dinosaurs include the Ornithopods. Pictured clockwise, starting from the upper left: Marasuchus, Archaeopteryx, Apatosaurus, and Allosaurus.

Scientists now think that some dinosaurs were endotherms (warm-blooded) due to the evidence that has been collected over the decades. There are still some scientists who do not agree, but the amount of evidence makes it likely. Some dinosaurs lived in polar regions where animals that needed sunlight for warmth could not survive in winter. Dinosaurs bones had canals, similar to those of birds, indicating that they grew fast and were very active. Fast growth usually indicates an active metabolism typical of endotherms. Dinosaurs had erect posture and large brains, both correlated with endothermy.

Rise of the Mammals

Mammals appeared near the end of the Triassic, but the Mesozoic is known as the age of the reptiles. In a great advance over amphibians, which must live near water, reptiles developed adaptations for living away from water. Their thick skin keeps them from drying out, and the evolution of the amniote egg allowed them to lay their eggs on dry land. The
amniote egg
has a shell and contains all the nutrients and water required for the developing embryo (
Figure
below
).

Amniotic eggs containing snake hatchlings.

Cretaceous Mass Extinction

Between the Mesozoic and the Cenozoic, 65 million years ago, about 50% of all animal species, including the dinosaurs, became extinct. Although there are other hypotheses, most scientists think that this mass extinction took place when a giant meteorite struck Earth with 2 million times the energy of the most powerful nuclear weapon (
Figure
below
).

An artist’s painting of the impact that caused the Cretaceous extinctions.

The impact kicked up a massive dust cloud, and when the particles rained back onto the surface they heated the atmosphere until it became as hot as a kitchen oven. Animals roasted. Dust that remained in the atmosphere blocked sunlight for a year or more, causing a deep freeze and temporarily ending photosynthesis. Sulfur from the impact mixed with water in the atmosphere to form acid rain, which dissolved the shells of the tiny marine plankton that form the base of the food chain. With little food being produced by land plants and plankton, animals starved. Carbon dioxide was also released from the impact and eventually caused global warming. Life forms could not survive the dramatic temperature swings.

You may be surprised to know that dinosaurs in one form survived the mass extinctions and live all over the world today. Birds evolved from theropod dinosaurs, and these creatures not only survived the asteroid impact and its aftermath, but they have also diversified into some of the most fantastic creatures we know (
Figure
below
).

Archeopteryx, the earliest known bird, lived during the late Jurassic.

Summary

Phytoplankton evolved to become the base of the marine food web.

In the Mesozoic dinosaurs filled the niches that mammals fill today.

Life of the Mesozoic appears to have ended with a giant asteroid impact.